In designing a portable hand held communication device, there are numerous problems to be solved to produce a competitive product. One significant concern is the electric power demand of the communication device. Since these devices are powered by electric storage batteries, reducing the device's power demand will allow the device to operate longer for a given battery capacity. This so-called "talk time" is a critical market feature.
In recent years digital communication methods have gained popularity and provided cost effective solutions to subscriber capacity problems, among other problems associated with analog systems. Of the various digital communication schemes, linear modulation schemes, such as quadrature amplitude modulation, provide good bits per hertz per second performance. Linear modulation techniques also provide other benefits, such as reduced spectral occupancy. However, a significant drawback to linear modulation is that power amplifiers for converting direct current (DC) power to radio frequency (RF) power for linearly modulated signals necessarily operate below peak efficiency. This poor efficiency operation is due to the fact that the amplifying device must be biased to an average output power level low enough to accommodate peak input signal levels without overdriving the amplifying device. If the amplifying device is overdriven, the output signal will be distorted. Consequently, it is not uncommon for linear amplifiers to have an average efficiency of 15%-20% in applications that have, for example, a 6 decibel (dB) peak power to average power differential. Even with such a poor average efficiency, however, the benefit of linear modulation for communicating digital information is considered a beneficial compromise of current drain goals. Furthermore, several methods have been developed to improve the efficiency of linear amplifiers.
Since the power demand of the transmitter power amplifier accounts for a majority of the power consumption when transmitting, it is preferable to increase the power amplifier efficiency. One method used to improve efficiency is called supply modulation. Supply modulation is performed by varying the supply voltage to the amplifying device in correspondence with the input signal or the average input signal, which keeps the bias condition of the amplifying device near an optimum level for power conversion efficiency. Supply modulation has been developed to an extent where the supply voltage tracks the input signal, but this is a very sophisticated, complex, and expensive solution. Thus, an input tracking method is not a preferable choice for a typical hand held communication device design. More commonly, for low cost applications, supply modulation is done in increments. In the incremental approach the supply is set to a predetermined level corresponding to a given range of input signal amplitude. This method has been used with limited success in handheld communication devices.
The two parameters which govern amplifier efficiency are supply voltage, as discussed briefly above, and the load resistance. Manipulating either of these parameters can result in better power conversion efficiency. The idea of varying amplifier load impedance to improve efficiency can be seen in a Doherty amplifier configuration. The Doherty amplifier uses at least two amplifier paths, a carrier amplifier and a peaking amplifier, and combines their outputs in such a way that the load impedance presented to the carrier amplifier decreases as the input signal power increases above a preselected threshold level, such that the saturation point of the carrier amplifier is not exceeded. A discussion of Doherty operation is presented in U.S. Pat. No. 5,420,541 entitled "Microwave Doherty Amplifier."
The Doherty topology has been used widely in sophisticated and high power applications, such as land based microwave communication systems and satellite communication systems. It has generally not been considered for low cost hand held consumer oriented communication devices because of the complexity involved. For one, the space occupied by having an additional amplifier path goes against the industry goals of small size and low weight. Secondly, the output matching networks are also physically significant. Because of the frequency at which many portable communication systems operate, the transmission line elements used in output matching are substantially long compared to the size of the communication device. Although such transmission line elements can be designed as folded elements, the volume and weight, added to the volume and weight of an additional amplifier path have made the Doherty approach undesirable for hand held communication devices. Therefore there exists a need for an efficient linear amplifier for hand held communication devices that does not have the full weight and volume requirements of a conventional Doherty amplifier approach.